This invention relates to the field of photonics, and in particular to a method and apparatus for measuring polarization dependent loss (PDL) in photonic devices, such as single-mode fiber optic components.
As a light wave propagates along a single-mode optical fiber, polarization-dependent attenuation occurs. This is known as polarization dependent loss (PDL). For many reasons it is important to be able to measure the PDL accurately for photonic devices, such as single mode optical fibers.
International standard IEC 61300-3-12, the contents of which are herein incorporated by reference, defines a method wherein a device under test (DUT) is illuminated by a small, typically four, set of well defined states of polarization. These measurements are followed by a matrix calculation to determine the PDL of the DUT.
One matrix calculation technique defined by the standard is known as the Mueller matrix technique. This technique provides an optical power representation of the performance of the DUT. The matrix representing the DUT and its optical properties is a square 16-element matrix, and the state of polarization of a light beam is described by a 4-element Stokes vector. The Stokes vector of the incident light multiplied by the Mueller matrix of the DUT gives the Stokes vector of the output light. Normally, the full Mueller matrix is not required to determine the PDL. The first row, which gives information on light intensity, is usually sufficient.
The problem with the Mueller technique is that it requires the four states of polarization to be accurately known. This is not always easy to achieve. In one method, the polarization light source generates three linear states of polarization and one circular. The circular state is generated by aligning a linear polarizer at 45xc2x0 to a quarterwave plate. Precise alignment of the quarter waveplate is essential to produce circularly polarized light. In addition, when the PDL is calculated from at a wavelength that is different from the design wavelength of the quarterwave plate, it has to be assumed that the retardance behaviour of the quarter waveplate is linear, an assumption that is not always true.
The PDL of a DUT can be calculated by measuring the transmission coefficient of the DUT for four known states of polarization, provided said states do not all lie in the same plane when depicted in the Poincarxc3xa9 polarization space. The PDL of the DUT is invariant under rotation of those four states of polarization provided the mutual dot products of the states are conserved. A dot-product conserving rotation is typically observed when states of polarization are launched in a single mode optical fiber: the states of the output are different from those at the input, but the mutual dot products are conserved. Displacing the fiber further rotates the output states of polarization but conserves the mutual dot products.
In the accordance with the principles of the present invention, the measurements are performed on a standard component with a known polarization dependent loss using a number of different dot-product conserving rotations, for example thirty, and the results optimized numerically. Four polarization states are initially assumed and the polarization dependent loss of the DUT calculated for each of the different dot-product conserving rotations. By minimizing the error relative to the known polarization dependent loss value of the standard component, it is possible to estimate the actual four states of polarization incident on the device and use these estimated states to calibrate the measurement device, which can then be used to determine the loss of a DUT having an unknown PDL.
According to the present invention there is provided a method of calibrating an apparatus for measuring the polarization dependent loss of an optical device under test, comprising generating at least four nominal polarization states of incident light with a multiple state polarization generator; passing the incident light through a substantially dot-product conserving polarization rotator at a first setting; measuring a transmission characteristic of the incident light through a standard optical component with a known polarization dependent loss value at the first setting of the polarization rotator for each of the polarization states; repeating the previous step for a plurality of different settings of the polarization rotator; for each setting of the polarization rotator, calculating a polarization dependent loss value for the standard optical component based on the measured transmission characteristics and the nominal polarization states; processing the calculated polarization dependent loss values to generate an aggregate error value for all the polarization rotator settings based on the known polarization dependent loss value; and minimizing the aggregate error value by adjusting the values of the nominal polarization states to estimate the actual polarization states produced by the multiple state polarization generator.
The aggregate error function is preferably of the form:   f  =            ∑              i        =        1            K        ⁢          xe2x80x83        ⁢                  (                              PDL            i                    -                      PDL            standard                    +          δ                )            2      
where PDLstandard represents the polarization dependent loss of the standard component, xcex4 represents a perturbation in the polarization dependent loss of the standard component due to the presence of connectors, and K represents the number of settings of said polarization rotator.
It will be appreciated by one skilled in the art that the object is to minimize the error function, but the function does not necessarily have to be at the absolute minimum value in order to estimate the actual polarization states with sufficient accuracy for measurement. The intent is to optimize the function as much as possible or as is required for a particular application.
Typically four states are employed to calculate the PDL of the standard component using the Mueller calculation method. More states could be employed, but no advantage would be gained. By estimating the actual polarization states in this way, the accuracy of measurement can be considerably improved.
The invention also provides an arrangement for calibrating a polarization dependent loss measuring apparatus including a multiple state polarization generator for generating at least four polarization states of a beam of incident light, comprising a light source for generating the beam of incident light; a dot-product conserving polarization rotator having multiple settings for performing different rotations on the incident light; a transmission characteristic measuring device for measuring a transmission characteristic of the incident light through the optical device for each of the nominal polarization states at each setting of the polarization rotator; and a processor programmed to calculate a polarization dependent loss value at each setting of the polarization rotator from the measured transmission characteristics and nominal polarization states, generate an aggregate error value for all the settings of the polarization rotator based on a polarization dependent loss value of a standard component, and minimize the aggregate error value by adjusting the values of the nominal polarization states to estimate the actual polarization states produced by the multiple state polarization generator.
The multiple state generator is typically a four state generator producing the following four nominal states: left circularly polarized, horizontally polarized, vertically polarized, polarized at 45 degrees.
Once the measurement apparatus has been calibrated, the PDL of an optical component under test whose PDL is not known can be calculated by the Mueller method, using all four states, or alternatively by another suitable method, for example, the Jones method, in which case only three states are needed for the actual measurement of the PDL of the optical component.
The processor may consists of two parts, the first part calculating the estimated actual polarization states and the second part forming part of the measuring device to determine the polarization dependent loss.